1. Field of the Invention
The present invention relates to a semiconductor electrode containing a phosphate and a solar cell employing the semiconductor electrode. More particularly, the present invention relates to a semiconductor electrode with improved power conversion efficiency through inhibition of recombination reactions of electrons, and a solar cell employing the semiconductor electrode.
2. Description of the Related Art
In recent years, numerous studies have focused on various alternative energy sources for conventional fossil fuels to solve urgent energy consumption problems. In particular, extensive research into effective utilization of natural energy resources, including wind power, atomic energy and solar energy, has been conducted to replace petroleum resources that may be used up within the next several decades. Solar cells take advantage of inexhaustible solar energy, unlike other energy sources, and are environmentally friendly. Since the first selenium (Se) solar cell was developed in 1983, silicon (Si) solar cells have drawn a great deal of attention and interest.
However, since silicon solar cells can incur considerable fabrication costs, there are some limitations in the practical application and improvement in the efficiency of the cells. To overcome these limitations, the development of dye-sensitized polar cells that can be fabricated at reduced costs is actively under consideration.
Unlike silicon solar cells, dye-sensitized solar cells are photoelectrochemical solar cells that consist essentially of photosensitive dye molecules capable of absorbing visible rays to form electron-hole pairs and a transition metal oxide for transferring the generated electrons. Various dye-sensitized solar cells have been developed. Of these, a representative dye-sensitized solar cell was reported by Gratzel et al. in Switzerland in 1991 (B. O'Regan, M. Gratzel, Nature 1991, vol. 353, p. 737). The solar cell developed by Gratzel et al. comprises a semiconductor electrode composed of titanium dioxide nanoparticles covered with dye molecules, a counter electrode (e.g., a platinum electrode), and an electrolyte filled between the electrodes. Such solar cells, which can be fabricated at low costs per electric power generated when compared to conventional silicon cells, are desirable as replacements for conventional solar cells.
The structure of a conventional dye-sensitized solar cell is shown in FIG. 1. Referring to FIG. 1, the dye-sensitized solar cell comprises a semiconductor electrode 10, a counter electrode 14, and an electrolyte layer 13 disposed between the semiconductor electrode 10 and the counter electrode 14, wherein the semiconductor electrode includes a transparent conductive electrode 11 and a light-absorbing layer 12 disposed between the transparent conductive electrode 11 and the electrolyte layer 13.
The light-absorbing layer 12 generally contains a metal oxide 12a and a dye 12b. The dye 12b may be in a neutral state (S), a transition state (S*) or an ionic state (S+). When sunlight is incident on the dye, the dye molecules undergo electronic transitions from the ground state (S/S+) to the excited state (S*/S+) to form electron-hole pairs, and the electrons (e−) in an excited state are injected into a conduction band (CB) of the metal oxide 12a to generate an electromotive force.
However, since the electrons (e−) in an excited state are injected from the dye molecules into the conduction band of the metal oxide 12a at a much higher speed than the electrons can migrate from the conduction band to the transparent conductive electrode 11, some of the electrons accumulate on the surface and the inside of the conduction band, where they recombine with the dye molecules or cause recombination reactions, e.g., bonding with redox couples present within the electrolyte, resulting in a decrease in power conversion efficiency. This decrease results in reduction of electromotive power. In particular, when the metal oxide layer is formed of nanoparticles, aggregation of the nanoparticles takes place, and interfaces formed between the nanoparticles act as resistors, resulting in a further decrease in electrical conductivity and power conversion efficiency.
Thus, inhibition of such accumulation and recombination reactions of electrons is considered significant in improving the electrical conductivity of electrodes to increase the power conversion efficiency of solar cells.